Proton transfer equilibria (BIH+ + B, e B , + B,H+)and NO+ transfer equilibria(B,NO+ + B, *B, + B,NO+) have been studied for the following bases B, benzene, toluene, o -, m -, and p-xylene. NO+ transfer equilibria for fluoro-and chlorobenzene have also been studied. Pulsed electron beam, high-pressure ion source mass spectrometry has been used to obtain the equilibrium constant K and hence the free energy changes AGO and from van't Hoff plots, AH0 and AS0. Entropy changes are in general much smaller for NO+ transfer than for H+ transfer but the magnitude of the changes in the proton affinity and NO+ affinity of toluene caused by a fluorine substituent is about the same, even though the absolute value of the proton affinity is greater by a factor of4. The position of the F substituent on toluene has a marked effect on proton affinity but noeffect on NO+ affinity. The latter appears to be responsive only to the inductive effect. [Traduit par le journal]

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